Weird problem with UDP and gevent

[Grant Edwards]

On 2013-10-18, James Harris <james.harris.1 at gmail.com> wrote:
> "Roy Smith" <roy at panix.com> wrote in message 
> news:l3riea$82$1 at panix2.panix.com...
>> I'm running:
>>
>> Ubuntu Precise
>> Python 2.7.3
>> django 1.4.5
>> gunicorn 0.17.4
>> gevent 1.0dev (rc3)
>>
>> I haven't been able to pin this down exactly, but it looks like if I
>> do (inside of a custom logging.Handler subclass):
>>
>>   # Paraphrased from the actual code
>> remote_addr = ("localhost", 9700)
>>  self.socket = socket.socket(type=socket.SOCK_DGRAM)
>>        payload = "..."
>> self.socket.connect(remote_addr)
>>        self.socket.send(payload)
>>
>> I get intermittant hangs in the connect() call.  If I rewrite this as:
>>
>> remote_addr = ("localhost", 9700)
>>        self.socket = socket.socket(type=socket.SOCK_DGRAM)
>>        payload = "..."
>>        self.socket.sendto(payload, remote_addr)
>>
>> everything works fine.  Has anybody seen anything like this?  I'm
>> guessing this is some kind of gevent bug.
>
> Those are two different things.

Yes.  They differ in the addresses _from_which_ packets can be
received. In the "connect" example, you will only be able to receive
packets from ("localhost",9700).  In the second example you can
receive packets from any address.  Since the snippets only send
packets not receive packets, they should both behave identically.

> You would normally use connect() on a SOCK_STREAM socket. It requires
> that the remote endpoint, in this case localhost:9700, has an open
> socket listening for connections. sendto() is the right thing to use
> with SOCK_DGRAM.

Either will work.  You can use connect() with a SOCK_DGRAM.  What it
does is 

 1) save the destination address and use it when you call send()

 2) limit the addresses from which packets will be received. 

>From the Linux connect(2) man page:

   If the socket sockfd is of type SOCK_DGRAM then addr is the address
   to which datagrams are sent by default, and the only address from
   which datagrams are received.

-- 
Grant